It's about that time of year where all the major US carriers shake up their mobile plans.

This year, the buzzword is "unlimited." Both T-Mobile and Sprint on Thursday introduced new plans touting unlimited talk, text, and data. On Wednesday, AT&T refreshed its offerings, loosening its policy on overage fees in the process. Verizon made similar changes in July, too.

Per usual, though, the big four have largely obfuscated what exactly their "unlimited" plans are doing. So let's break things down.

1/

T-Mobile

It costs $70 per month. If you want to add a second line, that'll cost $50 per month. After that, you can add up to eight more lines, with each of those at $20 per month. You can add a tablet to this as well, giving that unlimited LTE data for $20 per month.

All of this presumes you have your billing set to auto-pay; if not, each rate goes up by $5 per line.

There are caveats to the "unlimited" claim as well.

- Most notably, any video you stream over mobile data with the One plan will be capped at a standard definition resolution of 480p. If you want to be able to stream in higher definition (presumably 720p), you'll have to buy an "HD add-on" for $25 per month, per line. This is rough.

- If you want to use your phone as a mobile hotspot, that'll be unlimited, but only at 2G speeds, or 128 kbps. That's very slow! T-Mobile says you'll be able to add 4G tethering data, but it'll cost $15 per month for every 5GB you want to use.

- It'll still throttle some people's speeds, too. The carrier says users who consume more than 26GB of LTE data in a month "may see their data traffic prioritized behind other users." T-Mobile told Business Insider that there is no set speed for this prioritization, and claimed that affected users "won't notice a difference except during rare instances of congestion on a specific tower."

T-Mobile One will replace the carrier's current "Simple Choice Unlimited" post-paid plans. T-Mobile says it'll address prepaid rates at a later date. Customers on existing T-Mobile plans will be able to keep what they have now.

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John Moore/Getty

In exchange for the increased data cap, the One plan will cost more than the cheapest Simple Choice plan that's available today, which costs $50 per month for unlimited talk, text, and 2GB of data. A more appropriate comparison might be the next step up — that costs $65 per month for a fairly sizable 6GB of data with one line, and $120 per month with four.

Individually, the Simple Choice plan that already has unlimited LTE data costs $95 per month — the same as what it costs to have a One plan with the HD video add-on. It includes 14GB of 4G tethering data as well. A set of four pays $220 for unlimited data now — with One, they could pay $160 for SD video only (the same as a 10GB plan now), or up to $260 if they all wanted HD.

Interestingly, T-Mobile says the One plan will do away with the carrier's Binge On program, which allows approved apps to stream video over its network without counting against a user's data allowance. It's also raised numerous net neutrality concerns.

The carrier says existing Simple Choice users will still get Binge On, Music Freedom (essentially Binge On for music), and Data Stash (which rolls over unused data per month), but One will phase them all out. Formally, that is: In effect, the One plan makes those initiatives the norm, at least until you hit that 26GB point.

Cut through the noise and the numbers, and the One plan amounts to 1) a boon for those who need tons of mobile data and are okay with 480p video, 2) a price hike for many, 3) an annoyance for hotspot users, and 4) a minor change for those who already use an unlimited plan today. It goes live on September 6.

3/

Sprint

Much like T-Mobile, it's technically unlimited, but the "optimized" bit means video streaming will be capped at 480p. Online gaming will max at 2 mbps (which is slow), while music streams will top out at 500 kbps (which is good). It doesn't appear as if Sprint will offer an HD streaming option a la the One plan. The company hasn't responded to a request for comment. It will, however, offer 5GB of LTE tethering data before sending you down to 2G speeds.

The new plan starts at $60 per month for one line. A second will cost $40 per month. You can then add up to seven more, each at $30 per month. So a family of four would pay $160.

Again, Sprint already offers an unlimited data plan today. That's pricier — at $75 per month for the first line, and $45 per month for the second — and only includes 3GB of LTE hotspot data, but also doesn't have the restrictions this new plan introduces. (Currently, a $60 monthly plan gets you 12GB of "regular" LTE data.) The Unlimited Freedom plan will replace that offer on August 19. Sprint says its other existing plans will all stay intact.

4/

AT&T

Reuters/Shannon Stapleton

AT&T's new unlimited plan launched earlier this year. It starts at $100 per month for one user, and $40 per month for each additional line. There's a big catch, though: You need to buy one of the company's DirecTV or U-Verse TV subscriptions to even be eligible for it.

The carrier also says it "may slow data speeds" after 22GB of usage. That's a ton, but again, a plan with limits isn't technically "unlimited."

Outside of that, the carrier rearranged its more approachable "Mobile Share Advantage" plans earlier this week. Aside from adjusting its data buckets — lowering the cost of some of its higher-capacity options, but making it so its cheapest $30 data plan offers 1GB instead of 2GB — it also said that it's eliminating overage fees.

That means you won't be charged extra if you go over your data allowance. (Previously, you'd be charged $15 for every extra GB.) However, it also means that you'll be stuck at 128 kbps for the rest of your bill cycle. That could work in an emergency, but it's a glacial speed by today's standards.

5/

Verizon

When the carrier revamped its rates earlier this year, it introduced an option to waive overage fees called "Safety Mode," but that's only included in its more expensive plans. It, too, caps you at 128 kbps once you go over your limit. (Unless you pay another $15 just to bring it back to LTE speeds.) Anyone on a cheaper plan has to pay $5 just to get that.

6/

Unlimited still isn’t unlimited

The takeaway here? Mobile plans are still a mess. T-Mobile and Sprint's latest moves may sound appealing, but any discounts are quickly countered by genuine performance handicaps. The dream of a straightforward, mildly affordable, truly unlimited data plan is still just that: a dream.